Well, I cannot say it’s a frustrating activity, and yet there is a number of things worth mentioning :) In no particular order:
Ahn’Kiraj – Silithid Royalty doesn’t die like it should! Sometimes I need 5 natural resets before they give up and give me loot :) This bug trio is annoying!
Viscidus is nigh impossible to beat solo unless you’re a Frost DK, or scraping a dozen of specifically farmed items (something I’m not ready to do yet). Waiting for nerfs…
Make sure you run Blackwing Lair before Ahn’Kiraj – because goblin mobs there will drop Elementium Ingots. You need 6 to complete a couple of quests after C’Thun and get a number of weapon transmogs :)
Speaking of Blackwing Lair, the small dragon grounds are massively annoying to cross. Double that if you plan to go out to Blackrock Mountain. So God bless Bodhi Rana and his advice: after the last boss, queue a custom group, then quit it to teleport outside and to the nearest graveyard in 60 seconds!
Karazhan Chess Event is bugged, and it has as much as three of them! You can use </cancelaura Control Piece> macro in case a piece disappears (never happened to me though). You can talk to Medivh and reset the encounter to take off the Silence debuff after the game. The nastiest one is not solved: after you beat the event, the loot chest is not active, there’s no cog, and you can’t get loot or get it mailed to you after the dungeon. This one was not solved yet. Still, you can proceed to final boss.
Battle for Hyjal can be beaten easily, but it’s massively annoying for melee classes – because gargoyles and frost wyrms in the second part.
You need only one tower during Galakras encounter. Climb, clear, shoot him until Warlord Zaela “drops from the saddle”. Then just wait for him to attack you – may take a while.
Mantid boss in Siege of Orgrimmar is bugged! More often than not, Hisek will just stay on the platform and won’t go down. Pray he’s in the first group.
Archimonde in Hellfire Citadel is tricky on Mythic. He will heal to 50% when you go to Twisting Nether phase, and you’ll have to burn him down asap before he punches you flying to death. Barely scraped out my victory with a Holy Priest / 410 today after 3 attempts, and it required all possible buffs – intellect potion, rune, and food for Haste. Ofc, it will be easier for damage dealers, but all cooldowns should be saved for that phase.
Make sure you’re trying to overrun Lei Shen’s push at the longer platform edge, or else. The encounter is not wipe-safe :)
Make sure you’re blowing up Deathwing’s head armor pieces during Deathwing’s Spine! It’s something I double-check after my glorious half an hour spent, trying to blow up his ass armor plates :)
Wipe out all the existing trash in Molten Core – more often than not you will find set pieces in common mobs. In general, check the current raid’s set piece sources: if any of them says “world drop”, you should clear trash as a lawnmower.
Make sure to sell loot via Tundra Mammoth / Expedition Yak every time you have this opportunity. If short on bag space, take trash items (claws, eyes, goo etc.) and leave boss loot alone – it will be mailed to you afterwards!
Otherwise, older raids are a curb and stomp party :) Having fun with all classes!
Gnomecore 
First character you send to farm should always be warrior because they can learn all weapon models except wands. Second character should be cloth class – to cover wands.
For Viscidus, you can get Brunnhildar Harpoons for any character proficient with polearms. (https://www.wowhead.com/item=43600)
Molten Core trash pieces are boe and can be obtained on AH.
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Weapons are not my primary focus, but thanks! Actually going with both warriors nowadays :)
Viscidus’ problem is not just frost damage, but 30 physical hits in 30 seconds after it’s frozen. Not every class may pull out that. Whining on official forums is enormous, I hope nerfs are quite possible :)
Need to re-check the thing with Molten Core, but I’m under impression that I couldn’t send trash drop tier items to my other toons.
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You can use any guardian-summoning item (usually trinkets with wording “Use: calls forth XXX to fight at your side”, but there are also offhands with this effect) to add guardian’s attacks.
Trinkets with “call forth” in effects: https://www.wowhead.com/items/group-by:slot?filter=107:161;0:1;call%20forth:0
The most efficient should be voodooo mask from Zul’Aman (https://www.wowhead.com/item=69762) because it summons 3 guardians at once.
Also, unarmed attack with drums.
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And even more efficient is the Crate of Kidnapped Puppies (https://www.wowhead.com/item=86570) with 5 guardians summoned – but only if you already have it, as it drops *rarely* from Pandaria rare, kind of uncertain occurence, whereas Malacrass can be killed daily until you get the mask.
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For Silithid Royalty, kill one of them then wait few seconds, kill the second one and wait, then kill the third, you’ll never see the bug again :-)
For BWL small dragons grounds, you can skip them with the help of mobs after the second boss, pull them to the slow-mo ground to the left side where you see a big nest, stand there and they’ll knock you up to the second floor and near the 3rd boss, you should look for a video to see how it’s done.
Thanks for BWL tip, i always hearth by the end of it or logoff and play another character, then come back.
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BWL tip worked like a charm, couple of knocks up and you’re there with the third boss. And you can also jump down the same way when going back. Saved me 10 minutes!
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Silithid Royalty isn’t “bugged” per say, it’s just that one of the three tries to cast resurrection spell on other two, and with the damage a player does now, you need to kill them one-by-one, starting from the smallest and fastest one and leaving the “healer” for the last.
Similar situation with the Mantids in SoO: just be patient with your damage, use auto-attack if necessary.
BWL part with the whelps is frustrating on your way to the boss, sure (though some classes can negate the slowing effect to some point: ret. paladins has a seal that removes all slowing effects, mages have “Shimmer”,etc), but on the way back you can (and should) skip it completely by turning right after re-entering the zone and jumping down ouotside from the area of effect of the last slowing column. Actually, getting back to the entrance in that instance is super fast, imo.
Mammoth/Yak are “must have”, sure, but pets like Guild Herald or little orc/human from the Coliseum vendor in Northrend are also useful. Plus – many raids has vendors inside: Karazhan, Black Temple, etc.
Pro-tip: It is quite easier to get to the Blackrock Mountain for Horder chars nowadays, thank to the instant portals from Orgrimmar to Tirisfal Glades – then to Northern Stranglehorn – then a taxi to the Mountain.
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Silithid/Mantids: thanks, it’ll be nice to have a slaying plan rather than random luck :)
BWL: I’ll try jumping off. Actually I’ve only recently discovered Num Lock button which toggles automatic run, so I use it there and have time for a legitimate bio / beverage refill / play with a cat break :)
Using in-raid vendors by all means. Thanks for the pet tip!
Where’s the instant portal Tirisfal -> Stranglethorn? I never explored this option.
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The portal is on top of the right zeppelin tower in Tirisfal outside Undercity, you need “before BfA” phase, of course, so don’t forget to talk to the NPC. It used to be a zeppelin (same as from Orgrimmar to Tirisfal), but because lorewise Undercity is gone, those were removed and changed to portals for the old content players.
The portal to Tirisfal in Orgrimmar is on top of the tower as well, the one with the zeppelin to Howling Fjord.
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Found it on my own, it was fun! Had to guess about the phase without hints)
Correction: tower in Orgrimmar is about Eastern Kingdoms, so it holds zeppelin to Gromgol/Stranglethorn and portal to Tirisfal
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